Previous studies by Stewart and her colleagues have established that in rats rough and play is sexually dimorphic and that organizational actions of androgens are responsible for this sexual dimorphism. Their studies also indicate that either testosterone or dihydrotestosterone are responsible for organizing sex differences in play. Our own studies suggest that organizational influences of gonadal hormones extend at least to six days after birth but are completed by 20 days of age. Studies in progress or planned seek to: 1) refine the analysis of the temporal period within which hormones organize play; 2) conduct a systematic comparison of testosterone and dihydrotestosterone effects at different ages; 3) examine the effects of lesions of the amygdala, septum, basal hypothalamus and medial preoptic area on play to gain some notion of the brain mechanisms underlying the behavior; 4) study the role of olfaction in play by examining the effects of peripheral anosmia. Ultimately we seek to map the brain with neonatal hormone implants to determine where gonadal hormones are acting to exert their organizational effects. Adrenocortical hormones also modulate play; in males neonatal injections of corticosterone or dexamethasone profoundly depress play. We intend to examine this effect in both sexes at different ages. Brief periods of social isolation profoundly stimulate play in both sexes. Amphetamine injections (1 mg/kg) suppress play, both in group-housed and socially isolated animals. A pharmacological and behavioral analysis of this phenomenon is in progress.